Our objectives are severalfold: A. To produce radiation resistant mutants from a stable strain of mammalian cells growing in various environments of radiation. B. To identify, describe and assess biochemical, biophysical, and cytological differences between the mutants and the parent strain. C. To quantitate the overall mutation rate for radiation resistance as a function of various parameters such as dose rate, temperature, and time schedule of dose delivery including the simulation of a fractionated radiation therapy regimen, and to examine some of the kinetics of the production of specific mutations if warranted by the data obtained. The attainment of these primary objectives would sharply extend our understanding of radiation resistance and perhaps lead to new radiation modifiers in cancer radiotherapy. Further, quantitative information on the production of radioresistant mutants as a function of dose delivery could lead to new fractionation schedules designed to minimize radioresistant tumor regrowth during and after therapy.